METHOD FOR LIMITING THE CONCENTRATION OF OXYGEN CONTAINED IN A BIOMETHANE STREAM

Abstract

A process for producing biomethane by scrubbing a biogas feed stream, including introducing a feed gas stream into a pretreatment unit thereby partially separating a CO.sub.2 stream and an oxygen stream and producing a CO.sub.2-depleted gas stream which is then compressed to a pressure P1 above 25 bar abs; introducing the CO.sub.2-depleted gas stream to cryogenic separation in a distillation column to separate a nitrogen stream, the distillation column having n plates, n being an integer between 8 and 100; recovering a pressurized CH.sub.4-enriched stream by pumping the CO.sub.2-depleted gas stream) at a pressure P2 above 25 bar, wherein, when the molar concentration of nitrogen of the CO.sub.2-depleted gas stream is less than a predetermined threshold, nitrogen is injected prior to introduction into the distillation column, in order that the stream introduced into said column has a molar concentration of nitrogen at least equal to said predetermined threshold.

Claims

1.-11. (canceled)

12. A process for producing biomethane by scrubbing a biogas feed stream, the process comprising: introducing a feed gas stream into a pre-treatment unit thereby partially separating a CO.sub.2 stream and an oxygen stream and producing a CO.sub.2-depleted gas stream which is then compressed to a pressure P1 above 25 bar abs; introducing the CO.sub.2-depleted gas stream to cryogenic separation in a distillation column to separate a nitrogen stream, the distillation column comprising n plates, n being an integer between 8 and 100; recovering a pressurized CH.sub.4-enriched stream by pumping the CO.sub.2-depleted gas stream) at a pressure P2 above 25 bar, wherein, when the molar concentration of nitrogen of the CO.sub.2-depleted gas stream is less than a predetermined threshold, nitrogen is injected prior to introduction into the distillation column, in order that the stream introduced into said column has a molar concentration of nitrogen at least equal to said predetermined threshold.

13. The process of claim 12, wherein the distillation column comprises n real plates, n being an integer between 8 and 100, and wherein the CO.sub.2-depleted gas stream is introduced into the distillation column at the level of a plate between plate n-4 and plate n, plate n being the plate that is positioned the highest in said column.

14. The process of claim 12, wherein the predetermined threshold is equal to 5 mol %.

15. The process of claim 12, wherein P1 is greater than 50 bar absolute.

16. The process of claim 12, wherein the CO.sub.2-depleted gas stream comprises between 0.3 mol % and 2 mol % of CO.sub.2.

17. The process of claim 12, further comprising scrubbing the water from the gas stream compressed to the pressure P1.

18. The process of claim 12, wherein the separation of the CO.sub.2 and of the oxygen from the feed gas stream is performed by a unit comprising at least two separating membrane stages.

19. The process of claim 12, wherein the pressure P2 is greater than 40 bar abs.

20. The process of claim 12, wherein the CO.sub.2-depleted gas stream undergoes an expansion to a pressure P3 of between 15 bar abs and 40 bar abs prior to being introduced into the distillation column.

21. The process of claim 12, wherein prior to the expansion, the CO.sub.2-depleted gas stream is at least partially condensed in a heat exchanger.

22. The process of claim 12, wherein the CO.sub.2-depleted gas stream is at least partially condensed in a heat exchanger counter-currentwise relative to the CH.sub.4-enriched stream and to at least part of the nitrogen stream.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0068] For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

[0069] FIG. 1 illustrates a particular embodiment of a process according to the invention performed by a facility as represented schematically in the figure.

[0070] The same reference denotes a liquid stream and the pipe which conveys it, the pressures under consideration are absolute pressures and the percentages under consideration are molar percentages.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0071] In FIG. 1, the facility comprises a source of biogas (1) to be treated, a pretreatment unit (5) comprising a compression unit (2) and a CO.sub.2 and O.sub.2 scrubbing unit (23), a VOC and water scrubbing unit (3), a cryodistillation unit (4), and finally a methane gas recovery unit (6). All the items of equipment are connected together via pipes.

[0072] Upstream of the compression unit (2) is the CO.sub.2 scrubbing unit (23) and optional prior pretreatment units.

[0073] The CO.sub.2 scrubbing unit (23) combines, for example, two membrane separation stages. The membranes are chosen to allow the separation of at least 90% of the CO.sub.2 and about 50% of the O.sub.2. The retentate obtained from the first separation is then directed toward the second membrane separation.

[0074] The permeate obtained from the second membrane separation is recycled by means of a pipe connected to the main circuit upstream of the compressor. This step makes it possible to produce a gas (7) with less than 3% of CO.sub.2 and with a CH.sub.4 yield of greater than 90%. The temperature of this stream is typically ambient; if necessary, steps of cooling with air or with water may be incorporated.

[0075] The compression unit (2) is, for example, in the form of a piston compressor.

[0076] This compressor compresses the gas stream (7) to a pressure of between, for example, 50 and 80 bar. The stream exiting is denoted in FIG. 1 by the reference (8).

[0077] The (TSA) unit (3) for scrubbing VOC and water comprises two bottles (9, 10), They are filled with adsorbents chosen specifically to allow the adsorption of water and of VOCs, and their subsequent desorption during regeneration. The bottles function alternately in production mode and in regeneration mode.

[0078] In production mode, the bottles (9, 10) are fed with gas stream at their lower part. The pipe in which the gas stream (8) circulates splits into two pipes (11, 12), each equipped with a valve (13, 14) and feeding the lower part, respectively, of the first bottle (9) and of the second bottle (10). The valves (13, 14) will be alternately closed as a function of the saturation level of the bottles. In practice, when the first bottle is saturated with water, the valve (13) is closed and the valve (14) is opened to begin filling the second bottle (10). A pipe (15 and 16), respectively, emerges from the upper part of each of the bottles. Each of them is split into two pipes (17, 18) and (19, 20), respectively. The stream scrubbed of water and of VOC originating from the first bottle circulates in the pipe (18), whereas the stream scrubbed of water and of VOC originating from the second PSA circulates in the pipe (20). The two pipes are joined to form a single line (21) feeding the cryogenic unit (4).

[0079] In regeneration mode, the regeneration gas circulates in the pipes (17, 19). It emerges at the lower part of the bottles.

[0080] The cryodistillation unit (4) is fed via the pipe (21) in which circulates the gas stream (22) to be scrubbed. It contains three elements, a heat exchanger (24), a reboiler (25) and a distillation column (26), respectively.

[0081] The exchanger (24) is preferably an aluminum or stainless steel brazed plate exchanger, It cools the gas stream (22) circulating in the line (21) by heat exchange with the liquid methane stream (27) withdrawn from the distillation column (26). The gas stream (22) is cooled (28) to a temperature of about 100 C. The two-phase stream (28) resulting therefrom may alternatively ensure the reboiling of the reboiler of the vessel (25) of the column (26) and the heat (29) produced is transferred to the vessel of the column (26).

[0082] The cooled fluid (28) is expanded by means of a valve (30) to a pressure, for example, of between 20 bar absolute and 45 bar absolute. The fluid, which is then in two-phase form or in liquid form (31), is introduced into the column (26) at a stage E1 located in the upper part of said column (26) at a temperature, for example, of between 110 C. and 100 C.

[0083] The CO.sub.2-depleted gas stream (22) introduced into the column (26) at a stage E1 has an oxygen concentration equal to C1.

[0084] When C1 is strictly greater than 1 mol %, the process is stopped.

[0085] When C1 is strictly greater than 0.1 mol %, the gas stream (22) is introduced into the distillation column at a level E1 between plate n-4 and plate n, plate n being the plate that is positioned the highest in said column. When C1 is strictly greater than 0.5 mol % and less than or equal to 1 mol %, the gas stream (22) is introduced into the distillation column at a level E1 of plate n, plate n being the plate that is positioned the highest in said column.

[0086] The liquid (31) is then separated in the column (26) to form a gas (32) by means of the condenser (33). Cooling of the condenser (33) may be performed, for example, by means of a refrigerating cycle using nitrogen and/or methane. A portion (36) of the liquid (37) leaving the vessel of the distillation column (26), at a temperature of between 120 C. and 90 C., is sent to the reboiler (25) where it is partially vaporized. The gas formed (29) is sent to the vessel of the column (26).

[0087] The other portion (38) of the remaining liquid (37) is pumped by means of a pump (39) to form the liquid methane stream (27) which is vaporized in the exchanger (24) to form a pure methane gas product (40). This pumping step is performed at a high pressure, typically above the critical pressure and above 40 bar absolute, preferentially above 50 bar absolute. This pressure level makes it possible to avoid the accumulation of CO.sub.2 in the last drop to be vaporized of the exchange line. Since the gas is very low in heavy hydrocarbons, the dew point of the gas below the critical pressure is very low (typically below 90 C.).

[0088] The injection of nitrogen into the gas to be treated so as to limit the oxygen concentration in the distillation column thus makes it possible to solve the problem identified by the inventors of the present invention. Specifically, if the gas, with an equivalent oxygen concentration, contains more nitrogen, the risk of concentration at the top of the column becomes lower since the oxygen is more diluted in the nitrogen. A control system is thus put in place.

[0089] When the nitrogen concentration is above a content t1 (for example t1=5 mol %), no nitrogen is injected into the feed gas. And when the nitrogen concentration is below t1, nitrogen is injected into the feed gas so as to obtain a mixture with a composition approaching or even higher than t1 (typically, the injection rate is controlled as a function of the content in the mixture).

[0090] Since measurement of nitrogen in a gas is difficult directly, it is possible to use the measurement of methane of the gas, from which the oxygen and CO.sub.2 content is subtracted.

[0091] It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.